That’s because the metal bonds tightly to oxygen and it takes a lot of energy to break that bond. In essence, the process of making aluminum is a giant battery with the silvery metal being reduced to purity at the cathode while oxygen bonds with the carbon anode to make, you guessed it, CO2. It takes roughly 15 kilowatt-hours of electricity to make just one kilogram of aluminum via electrolysis.

“What’s a big current sink? Aluminum smelters,” explained Luis Ortiz, research director for materials scientist Donald Sadoway at the Massachusetts Institute of Technology (M.I.T.), at last week’s ARPA-E summit. “Maybe the aluminum industry is sitting on the answer [to large-scale electricity storage] all along.”

Unfortunately, thanks to that CO2 gas floating away, this aluminum-making process turns out to be a very bad battery—it is simply not reversible. But David Bradwell, a graduate student in Sadoway’s lab, tweaked the process to involve liquid metals for the anode, cathode and even electrolyte and evolved a battery that has a high charge transfer, long life and costs “below $100 per kilowatt-hour,” according to Ortiz. “It is as good or better than lithium ion in energy density but cheaper than lead acid.”